It is well known that the addition of rubber particles to brittle plastics can result in increased impact resistance of the resulting rubber-brittle plastic blends relative to that of the unmodified brittle plastic It is, however, very difficult to design polymer blends which have good clarity and weatherability in addition to impact resistance. For example, butadiene rubber modified polymethylmethacrylate (PMMA) has better impact resistance than unmodified polymethylmethacrylate. But, the performance of such products rapidly declines on exposure to sunlight, even when stabilizers are added to minimize this degradation. Styreneacrylic modified PMMA, in contrast, exhibits better weather-ability but poor cold temperature impact resistance.
The use of saturated rubbers, e.g. ethylene-propylene copolymers or hydrogenated polyisoprene, in place of butadiene rubber results in a product which is more stable in an outdoor environment. However, the high level of saturation of these rubbers, which results in improved weatherability, makes it very difficult to chemically modify the rubber. Controlled chemical modification of the rubber is essential in the development of a weatherable, impact resistant, high clarity rubber modified PMMA. This has not been accomplished until the present invention.
The high clarity and luster of polymethylmethacrylate are among the principal advantages of this polymer over many other commercial systems. There is a need for a toughened polymethylmethacrylate with good clarity and weatherability, for example, in the manufacture of exterior signs and decorative materials.
In the standard processes for the preparation of impact resistant brittle polymers, highly crosslinked, discreet rubbery particles are deliberately prepared prior to final processing This type of process suffers from two major drawbacks. First, relatively high levels of rubber are required for substantial impact performance, i.e., there is relatively low rubber efficiency. Second, the rubber particles can be adversely affected during processing, further lowering the rubber efficiency. It would be advantageous if the formation of discrete particles could occur much later in the process.
It would also be advantageous if a concentrated rubber containing composition could be prepared and stored. This material could be blended with pure polyacrylate to form finished products where and when desired. This would result in savings in shipping, handling and storage costs.